Steroid hormones act via specific receptor proteins to exert profound effects on the development and physiological regulation of virtually every animal tissue. We propose in this application to investigate the molecular biology of glucocorticoid action in tissue culture lines derived from mouse lymphosarcoma and mammary carcinoma cells. The lymphoid cells, which are normally killed by glucocorticoids, would be used primarily to develop genetic mutants defective in the steroid response. These mutants would be used to identify the molecular components involved in hormone action, to define complementation groups by somatic cell hybridization techniques, and to act as genetic controls in correlating biochemical observations with in vivo events. The mammary cells respond to glucocorticoids with increased production of mammary tumor virus RNA. Using specific cDNA probes to directly quantitate viral transcripts by RNA-DNA hybridization, we shall carry out detailed studies of the hormonal effects on the metabolism of this specific gene product. Systems for in vitro transcription of viral RNA will be developed, using chromatin and nuclei. Ultimately, such systems will be useful in reconstructing elements of the steroid-mediated induction process from purified components. In addition, proviral DNA will be isolated as free intermediates or in the host genome integrated form in a attempt to detect specific DNA binding sites for steroid receptors. These approaches may also provide new information on the role of hormones in specific cell death, and the regulation of viral carcinogenesis.